The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
A hard disk drive (HDD) control system typically positions a read/write head over a disk medium by locking a servo loop to a predetermined servo positioning pattern (or servo wedge). FIG. 1 illustrates an example servo wedge 21. The servo wedge 21 includes a preamble field 22, a servo sync-mark (SSM) field 24, a track/sector identification (ID) field 26, and a plurality of position error signal (PES) fields 28-1, 28-2, . . . , 28-N (referred to collectively as PES fields 28). The servo wedge 21 further includes a plurality of repeatable run out (RRO) fields 30-1, 30-2, . . . , 30-M (referred to collectively as RRO fields 30). The RRO fields 30 generally provide head positioning information to compensate for RRO disturbance of a hard disk drive.
Typically, an acquisition pattern is recorded in the preamble field 22. The acquisition pattern enables a read/write channel within an HDD to acquire appropriate timing and amplitude information from a read signal before reading servo information. The preamble field 22 is used to lock a servo timing loop's clock phase and frequency to a servo wedge and synchronize servo information stored in the servo track/sector ID field 26. The SSM 24 is used to mark the ending point of the preamble field 22 and the starting point of the track/sector ID FIELD 26. The SSM 24 is also used as a reference point for the position of other data payloads throughout servo fields.
The track/sector ID field 26 indicates both a circumferential position and a coarse radial position of the head. The track/sector ID field 26 typically includes a servo track number that identifies a current track that the head is over while the head is seeking to a selected track. The track/sector ID field 26 further includes sector identification data used to identify data sectors of the tracks.
The position information included in a servo field is generally used to determine a fine position of the head on the disk surface and to provide an HDD control system an instantaneous position error signal. The PES fields 28 provide information concerning fine radial positioning of the head. The servo system may compare the relative signal strengths of various PES fields to determine the fine position of the head on the disk surface.
Conventionally, the RRO fields 30 are stored on the disk itself. An exemplary storage method embeds the RRO fields 30 within the servo wedge 21. The preamble field 22, SSM 24, track/sector ID FIELD 26, and PES FIELD 28 fields may be written at the same time. Hence, these fields are generally phase coherent (i.e. minimal phase shift is present between the fields). The RRO fields 30 are typically written at a different time due to various requirements of calibration techniques used in HDD manufacturing processes. Unavoidable circuit latency and physical structure of the disk drive read/write head can cause written RRO fields 30 to have a phase offset with respect to the servo preamble field 22, SSM 24, and PES fields 28. RRO field detectors may be sensitive to such phase offset, and, therefore, RRO field detection may be degraded if phase offset is present.
Referring to FIG. 2, an exemplary relationship between detector bit error rate and phase offset is shown. Phase offset can cause detector performance degradation since more error bits will be present in detected data. A small phase offset, even when significantly less than one user bit, can dramatically increase bit error rate. For example, the phase shift indicated at 31 results in a bit error rate approaching 100%.
Referring back to FIG. 1, phase offset due to RRO writes may require each RRO field 30 to have an RRO preamble 33. The RRO preamble 33 is used to adjust the phase of the detector to allow for reliable reading of remaining portions of the RRO field. Further, the RRO preamble 33 may be used to bring the detector to a known state after the detector receives unknown values from an unwritten portion of the disk medium (or magnetic medium) after the final PES field 28-N and before the RRO preamble 33. An RRO sync-mark (RROSM) 34 is placed after the RRO preamble 33 to act as a reference point for the position of an RRO data payload 35. An RRO pad 36 is used to manage intersymbol interference (i.e. the interference between the current bit and previously read channel bits).
The acquisition of previously written data in a system with intersymbol interference may be described by the following equation:
      y    ⁡          (      n      )        =            ∑              i        =        0            m        ⁢                  ⁢                  c        ⁡                  (          i          )                    ×              x        ⁡                  (                      n            -            i                    )                    where y(n) represents an acquired signal, x(n) represents previously written data, and c(i) represents a channel coefficient that weights the written data according to an amount of intersymbol interference. The effect of the unwritten portion of the magnetic medium, between the final PES field 28-N and the first RRO field 30-1, is also described by the equation. Assuming that x(0) to x(n) represents written data, the value of x(k) for k<0 represents the unwritten portion of the magnetic medium containing unknown values. The values of y(0) to y(m−1) are therefore unknown. Consequently, the written data from x(0) to x(m−1) is not accurately retrievable.
Referring to FIG. 3, a five-bit sequence 40, such as may be included in one of the RRO fields of the servo wedge 21, is shown. The five-bit sequence includes five user bits 42, each separated by a boundary, such as that shown at 44. Each of the user bits 42 may include multiple channel bits. For example, the fourth user bit is shown with four channel bits a, b, c, d, where channel bit b is indicated at 46. Each channel bit 46 may represent sampled analog data obtained from the magnetic medium. Phase offset may be measured in units of user bits 42 and/or channel bits 46.
Referring to FIGS. 4A-4C, various detection scenarios are possible when an RRO detector acquires RRO data. A portion of an RRO field, including five user bits 42, is shown for each scenario. Referring to FIG. 4A, the RRO detector may be out of phase with the RRO field, which may result in a high bit error rate. The phase error is indicated at 50. The RRO preamble 33 is added to mitigate this problem. The RRO preamble 33 may include a sinusoidal waveform, which may be formed from alternating 1's and 0's, to provide a reliable phase reference. The typical length of the RRO preamble is 6 to 12 user bits.
Referring to FIG. 4B, the RRO detector is shown adapting to the phase of the RRO field. At reference numeral 52, the RRO detector has aligned its phase with the bit boundaries of the RRO field. In addition, there is no position offset between the RRO field and the RRO detector. In other words, the RRO detector reads bit 4 as bit 4 and bit 5 as bit 5.
However, as shown in FIG. 4C, a position offset may be present after phase adjustment has been completed. At 54, the phase of the RRO detector is aligned with the bit boundaries of the RRO field. However, bit 5 will be read as bit 4, bit 6 will be read as bit 5, etc. Depending on the phase alignment process, even greater position offsets may be possible.
To allow the RRO detector to eliminate position offset, the RRO sync mark (RROSM) 34, a predetermined bit sequence, is added. When the RRO detector finds that bit sequence, the RRO detector will have established the position from where data is being read. RRO data, such as the RRO data payload 35, can then follow the RROSM 34. The RRO detector may be programmed to interpret the data immediately following the RROSM 34 as the RRO data payload 35. The typical length of the RROSM 34 is 2-7 user bits. The RRO preamble 33 and the RROSM 34 therefore allow the RRO detector to correct for phase offset and then for position offset. An RRO pad 36 may be added to the end of the RRO data payload 35 to mitigate intersymbol interference.
Referring to FIG. 5, a flow diagram illustrates steps performed in removing phase offset and positional offset. Control begins in step 60, where phase adaptation is performed. After phase adaptation is complete, or after a predetermined period of time, control continues in step 62. The predetermined period of time may correspond to an upper limit of time needed to successfully perform phase adaptation.
In step 62, a timer is started. Control continues in step 64, where control begins to look for the sync-mark bit pattern. Control continues in step 66, where control determines whether the sync-mark has been found. If so, control transfers to step 68; otherwise, control transfers to step 70. In step 68, the RRO payload is read immediately following the sync-mark. Control then ends. In step 70, control determines whether the timer has expired. If so, control ends; otherwise, control returns to step 64 to continue looking for the sync-mark.